About 93 million years ago, a burst of volcanic activity on the ocean floor led to a massive extinction event that killed one-tenth of the world’s marine invertebrates, according to a new study. The Caribbean region was the likely source of the sea-floor eruptions, says [study coauthor Steven] Turgeon. He says massive amounts of lava would have burbled and blasted up from inside the earth, setting off a “chain reaction” that took thousands of years to play out [Canwest News Service].

[T]he volcanoes spewed out metal-rich fluids that seeded the upper level of the ocean with micronutrients…. Tiny life forms on the sea surface, called phytoplankton, gorged on the food, and storing up carbon as they grew. They then sank to the sea floor and decayed, stripping the ocean of oxygen [BBC News] in a large-scale example of the phenomenon that causes the annual “dead zone” in the Gulf of Mexico. The oxygen-starved waters were a poor environment for marine life, and creatures from giant clams to tiny invertebrates went extinct.

Researchers had long known that a loss of oxygen in the world’s oceans occurred during the Cretaceous Period, a balmy era when palm trees grew in Alaska and large reptiles paddled in the warm waters of the Arctic Ocean. But until this new study, published in the journal Nature [subscription required], they didn’t know the cause of the so-called “oceanic anoxic event.”

Turgeon’s team gathered evidence from sedimentary rocks that were undersea during the Cretaceous Period, and found a rock layer marked with a metallic isotope found in the magma spewed forth in a volcanic eruption. Above that, researchers found other layers of rock such as a thick black shale layer off the northeastern coast of South America that contains as much as 23 percent organic matter [Science News], indicating a massive die-off of marine organisms. Today the layer of organic matter… makes up nearly a third of present-day recoverable oil reserves, Turgeon said [National Geographic].